It has long been known to recover energy and/or combustible gas by partially combusting fuel in a reactor, and many different methods and processes for such recovery are known from the literature, as are also many different reactor designs for putting these processes into effect. Gasification of coal, recovery of combustible gas from waste and biomaterials of different origins, and gasification of spent liquors from the pulp industry are some examples of known applications. These processes have acquired considerable economic significance. Such methods and processes are also highly significant from an environmental aspect, since they enable waste which would otherwise be discarded and contaminate the environment to be put to a useful purpose.
In the pulp industry, different spent-liquor gasification processes, and then particularly black-liquor gasification processes have obtained more and more importance as a replacement for the earlier known combustion processes, such as the well known Tomlinson process. The recovery of energy can be kept separate from the recovery of process chemicals, by controlling the gasification process in a suitable manner and the process can also be controlled to recover sulphur in a gaseous phase and the base, for instance sodium carbonate, in a solid or molten phase. Such processes are described, for instance, in U.S. Pat. Nos. 3,073,672, 3,333,917 and 4,872,950.
It is often desirable to carry out the gasification processes at a pressure above atmospheric pressure, since this will provide several process-technical advantages. For instance, the reactor used may be given smaller dimensions while the throughput is not changed. Furthermore, the pressure can be used as a process control parameter, so as to make it possible to work at a higher temperature when using a higher pressure and to enable the process to be controlled in the desired direction. This is described, for instance, in the aforesaid U.S. Pat. No. 4,872,950.
However, the use of elevated pressures in fuel gasification processes is encumbered with several problems. When working at pressures of about 3 bars and thereabove, the volume of the reactor is normally so small as to make it difficult to maintain a good fluidization of the reacting material in the system. Furthermore, the time available for the gasification reaction is often much too short in a continuously working reactor. This results in an impaired efficiency and in a poor process economy. Poor efficiency also results in a higher load on subsequent purification apparatus and also in larger quantities of waste The present invention eliminates these problems to a great extent.